Abstract: 50 年前，疲劳设计在美国人看来并非是一个很严重的桥梁性能问题。疲劳设计运用了限制最大应力的概念，即 假定疲劳极限发生在2 × 106 次循环荷载时，利用最小应力与最大应力的比R，限制各点的最大应力。根据道路类型与 每日平均车流量，采用型号为HS-20 的设计车来测定特定周期。在20 世纪60 年代末至70 年代初，发现钢结构桥梁中 存在疲劳裂纹。1967—1987 年，针对焊接部位进行了大量的梁试验。根据试验结果，统计得到了数据库，并认为应力 幅度才是唯一主要的设计应力。1974 年，美国AASHTO 规范采用了应力幅度设计疲劳强度，这意味着只有循环活荷 载才是重要的。基本疲劳强度曲线提供了疲劳寿命下限设计方法，该方法是当今世界广泛应用的方法。基于钢桥的 经验认识到: 尽管采用不在受拉翼缘设计焊接，但由于平面扭曲也会在腹板中产生大量的裂纹，而且这些裂纹由于受 到三向应力的作用，即使在疲劳裂纹没有增长的情况下，在过去10 年里也造成了意外的脆性断裂。

Key words: 桥梁, 钢材, 设计, 连接, 疲劳, 应力幅度, 裂纹, 焊接, 试, S － N 曲线, 扭曲, 脆性断裂

摘要： Fifty years ago，fatigue design was not considered a serious bridge performance issue in the US． Fatigue design utilized the concept of limiting the maximum stress at details by using the R-ratio of minimum/maximum stress and an assumption that the fatigue limit occurred at 2 million cycles． The HS-20 design truck was used for specific cycles based on the type of road and the average daily truck traffic ( ADTT) ． Fatigue cracks were observed to develop in steel bridges in the late 1960’s and early 70’s． This development and the results of large scale beam tests on welded details that were carried out between 1967 and 1987 provided a statistically sound data base and established that stress range alone was the only statistical significant design stress． The fatigue provisions adopting stress range were changed by AASHTO in 1974． This meant that only the live load cyclic stress was important． The basic fatigue resistance curves provide a log normal lower bound design basis for fatigue life that is used world wide today． Experience with steel bridges also showed that a rule adopted after WWII which avoided welding to tension flanges，has resulted in extensive web gap cracking from out of plane distortions． During the last decade these small web gaps have resulted in unanticipated brittle fractures as a result of tri-axial stresses even in the absence of fatigue crack growth．

关键词: bridges, steel, design, connections, fatigue, stress range, cracking, welding, tests, S-N curves, distortion, brittle fracture